Absorbent structures comprising a capillary absorbent substrate and having an osmotic absorbent applied thereto are known in the art. As used herein, a "capillary" absorbent structure absorbs liquids, such as water, by capillary attraction of the liquids due to the thermodynamic force of attraction between a liquid and the solid surface of the capillary medium. In contrast, as used herein, an "osmotic" absorbent structure absorbs liquids deposited thereon by equalization of differential partial fluid pressure in the absence of ion exchange, forming a gelatinous substance which imbibes the liquids. As used herein, an "absorbent structure" refers to materials which, in combination, absorb liquids by both osmotic and capillary absorptions.
The capillary absorbent may be provided in the form of a substrate, for the osmotic absorbent to be later applied thereupon. Typically the capillary absorbent substrate is a generally planar, almost two-dimensional material, such as paper, nonwoven fabric, woven fabric, or even formed film, having two principal directions and a thickness in the third direction.
As used herein, the X-Y plane refers to the plane of the absorbent structure 20 as it is laid flat on a horizontal surface. The Z-direction is the direction extending outwardly from and orthogonal to the X-Y plane.
The osmotic absorbent may be made of acrylic acid, starch grafted acrylate copolymers, etc. Such osmotic absorbent materials are commonly used as absorbent gelling materials or superabsorbers in disposable absorbent articles such as diapers and sanitary napkins. The osmotic absorbent may be applied to the substrate in the form of a liquid precursor, to be later cured into an osmotic absorbent.
The osmotic absorbent material may be applied to the capillary absorbent substrate as a liquid precursor, such as a liquid monomer, then crosslinked to form an absorbent polymeric material. Usually the liquid precursor is applied to the capillary absorbent substrate in a liquid form and typically comprises some form of acrylic acid.
Typically, the liquid precursor is applied to the absorbent substrate by spraying, impregnation, etc. to provide a uniform coating thereon. Other teachings in the art suggest discontinuous applications of the liquid precursor to the substrate through brushing, roller coating, etc. Once the liquid precursor is applied to the capillary absorbent substrate, the liquid precursor may be crosslinked through elevated temperature, irradiation, etc.
Examples of such attempts in the art include U.S. Pat. No. 4,008,353 issued Feb. 15, 1977 to Gross et al., U.S. Pat. No. 4,061,846 issued Dec. 6, 1977 to Gross et al.; U.S. Pat. No. 4,071,650 issued Jan. 31, 1978 to Gross; U.S. Pat. No. 4,835,020 issued May 30, 1989 to Itoh et al.; U.S. Pat. No. 4,842,927 issued Jun. 27, 1989 to Itoh et al.; U.S. Pat. No. 4,865,886 issued Sep. 12, 1989 to Itoh et al., U.S. Pat. No. 4,892,754 issued Jan. 9, 1990 to Itoh et al.; U.S. Pat. No. 5,079,032 issued Nov. 21, 1988 to Miyake et al. and Great Britain Patent 1,452,325 published October, 1976 in the name of Triopolis.
However, deposition of an osmotic absorbent onto a capillary substrate according to the prior art can present serious problems. For example, typically the osmotic absorbent is introduced to the substrate as a liquid precursor, as noted above. The liquid precursor, of course, flows by capillary attraction throughout the entirety of or throughout particular regions of the capillary substrate onto which the liquid precursor was deposited, until the liquid precursor is absorbed. The liquid precursor is then cured into an osmotic absorbent which is dispersed throughout the capillary substrate in a three-dimensional matrix.
When a liquid insults the absorbent structure, the osmotic absorbent begins to swell upon imbibing the liquids. However, the osmotic absorbent is constrained from swelling by the three-dimensional fibrous matrix of the capillary substrate. If osmotic absorbent is unable to swell--due to the constraints of the fibers in the capillary substrate--the osmotic absorbent will not reach its full absorptive capacity. The liquids intended to be absorbed by the absorbent substrate may breach the perimeter of the absorbent substrate, resulting in leakage.
Further liquid insults will not be absorbed by the absorbent structure, because the osmotic absorbent cannot be utilized to its full capacity. Instead, additional liquid insults will run off the absorbent structure and breach its perimeter.
Absorbent liquids running off or breaching the perimeter of the absorbent structure can be particularly disastrous if the absorbent structure is used in the core of a disposable absorbent article, such as a diaper or a sanitary napkin. Osmotic absorbents, capillary absorbents, and combinations thereof have long been used in disposable absorbent articles. However, with the advances in this art, the consumer has come to expect disposable absorbent articles which rapidly absorb liquid insults and do not leak under normal wearing conditions.
Accordingly, it is an object of this invention to provide an absorbent structure having osmotic and capillary absorbing capabilities. It is further an object of this invention to provide such a structure wherein the osmotic absorbent is not constrained from swelling by a capillary substrate upon imbibing liquids. It is further an object of this invention to provide an absorbent structure having these characteristics and which provides new benefits not previously known in the art for use in disposable absorbent articles, particularly maintaining the pattern of an osmotic absorbent on a capillary substrate during the life of the disposable absorbent article.